ANALYTICAL CHEMISTRY
1656 As can be seen from Figure 2, at a concentration of 30% rubber hydrocarbon by volume a change of 0.0001 in refractive index reading of the mixture of rubber in the 1-bromonapht,halene n-ill cause a change of 0.35% in the results. Very clear and sharp refract,ometerlines are usually obtainable from the mixture, a.nd the mean of one series of observations very seldom differs from the mean of another series o n the same sample by more than 0.0001. An error of O.OOO1 in index for the 1-bromonaphthalene solvent will produce a n error of about 0.Z70. The error should not be so large as this if the solvent is stable a t 140’ C. An error of 0.0001 in index for the rubber hydrocarbon produces a n error of only 0.1%. T h e precision of the determination of rubber hydrocarbon could be improved if a suitable solvent could be found which had a n index higher than t h a t of I-bronionaphthalene. As i t is probably not practical to increase appreciably the precision of the refractive index measurements, the most promising means for increasing the precision and accuracy of this method is t o find a solvent with a higher refractive index. The refractive index method makes use of equations involving only measured physical constants and no empirical correcting terms. The chromic acid oxidation method of Burger, Donaldson, and Baty (’7) and the rubber bromide method of Willits, Swain, and Ogg (12)both make use of empirical factors in the calculation of the rubber hydrocarbon content. Gowans and Clark ( 8 ) )however, have recent’lyreported a n improvement in the bromination procedure almost t o the point where the theoretical factor is applicable. The chromic acid oxidation method can be applied t o either unvulcanized or vulcanized rubber and also to mixtures of natural and synthetic rubbers. Unfortunately, the rubber bromide and the refractive index methods are applicable only t o unvulcanized natural rubber. The precisions of the bromide and the refractive index methods are about, the same, but both these methods give better precision than the chromic acid oxidation method. The refractive index procedure is less complicated, and requires a smaller proportion of the operator’s time. Its greatest asset, however, is the ease with which a large number of determinations can be made a t the same time. This technique may be found applicable not only to natural
rubber but also t o synthetic polyisoprenes and other solvents. However, the physical constants for the materials and also the validity of the assumptions must be carefully checked as the isomeric forms of the units in the polyi~oprenesmay influence the constants from sample t o sample. L l T E R T U R E ClTED
(1) Ani. SOC.Testing Materials, Conmiittee D-11 on Rubber and Rubberlike Materials, “A.S.T.M. 1949 Standards,” Part 6, p. 822. (2) Ibid., p. 840. (3) Arnold, Aurelia, Madorsky, Irving. and Wood, L. A4.,- 4 w . k ~ .
CHEM.,23, 1656 (1951). Bauer, Ti., and Fajans, K., Chap. S X in A. Weissberger’s “Physical Methods of Organic Chemistry,” 2nd ed., Kew Tork Vol. I, Part 11, p. 1141, Interscience Publishers, 1949. (5) Bekkedahl, Sorman, Am. SOC.Testing Materials, Spec. Tech. Pub., 74,74 (June 1947): Rubber Age (Ar. Y.), 62, 173 (1947). (0) Bekkedahl, Norman, J . Research Katl. B u r . Standards, 43, 145 (1949). (7) Burger, V. L., Donaldson, W. E., and Baty, J. A , , Ani. SOC. Testing Materials, Bull. 120, 23 (January 1943); Rubber Chem. and Technol., 16, 660 (1943). (8) Gowans, IT. J., and Clark, E’. E., paper presented before 58th Meeting, Division of Rubber Chemistry, ~ M E R I ~ . . ~ .(.IHE>IIC.AL ?SOCIETY, Washington, D. C., March 2, 1951. (9) McPherson, A. T., Bur. Standards J . Research, 8 , ‘751 (1932): Rubber Chem. and Technol., 5 , 523 (1932). (10) McPherson, A. T., and Cummings, A. D., J . Research .Vatl. B u r . Standards, 14, 553 (1935); Rubber Chem. and Technol., 8 , 421 (1935). (11) Smith, W.H., Saylor, C . P., aiid Wing, H. J., B u r . Standard8 J . Research, 10, 479 (1933); Rubber Chem. and Technol., 6, 361 (1933). (12) Willits, C. O., Swain, 11. L., and Ogg, C. L., IND. ENG.CHEM., AN.~L.ED., 18, 439 (1946); Rubber Chem. and Technol., 20, 320 (1947). (13) Wood, L. A , Proceedings of Rubber Technology Conference, London, England, May 1938, p. 933, Cambridge, England, \V, Heffer and Sons, Ltd.; Rubber Chem. and Technol., 12, 130 (1939). (14) Wood, L. A., and Tilton, L. W., Proceedings of Second Rubber Technology Conference, London, June 1948, p. 142; J . Research ;\‘atl. B u r . Standards, 43, 57 (1949); Rubber Chem. and Technol., 23, 661 (1950). (4)
RECEIVED April 13, 1451. Presented before the 58th Meeting of the Division of Rubber Chemistry, AVERICAX CHEMICALSOCIETY, W’aahington, D . C., March 2 , 1951.
Measurement of Refractive Index of Elastomers AURELIA ARNOLD, IRVING MADORSKY’, AND LAWRENCE A. WOOD National Bureau of Standards, Washington, D. C .
F
It011 a careful study of the factors involved in tlie niearui’ement of t,he refractive index of elastomers, especially GR-S
synthetic rubbers, by means of a n -4bbe-type refractometer, a procedure for measurement has been developed which takes advantage of the inherent adhesiveness of most elastomers. This adhesiveness enables the polymer t o ‘ k e t ” the prism of the refractometer without t,he use of a contact liquid. The principle of the method is based on work done some years ago on natural rubber by RlcPhereon and Cummings (6). The detailed procedure has been developed in recent years by a number of different workers a t the Sational Bureau of Standards. Although the method, as given in this paper, has been described only in report form ( 2 , 7 , 8 ) , it has been used for several years as a basis for the determination of bound styrene in copolymers of butadiene and styrene ( 7 , l l ) ,and is currently employed for control purposes in the government-owned synthetic rubber plants. It has also been used by other investigators in deter1 Present address, Johns Hopkins Applied Physics Laboratory, Gilrer Spring, M d .
mirnng the per cent of rubbei hJdrocarbon in natural rubber (j), and for locating the second-order transition temperatures of natural and various synthetic rubbers (IO). APPARATUS
The conventional Abbe refractometer, graduated to the third and read t o the fourth decimal place, is used for the actual measurements. Water from a rrservolr a t room temperature is circulated through the housing of the measuring prism and through a cored brass block of about the same length and width a s the prism housing. The temperature of the circulating water is the same as t h a t nf the specimen only when this temperature is near t h a t of the ambient air. Consequently, it is preferable to allow the water temperature t o be approximately that of the room and t o reduce observations t o a standard temperature by calculation rather than t o maintain the water a t the standard temperature in a room t h a t may be a t a n appreciably different temprrature. T h e refractometer should be equipped with a thermometer graduatrd in units of 0.2” C. or less. If the temperature is not known to about this accuracy, errors will be introduced in the fourth decimal place of index. The standard glass test pieces furnished with refractometers
V O L U M E 2 3 , NO. 1.1, N O V E M B E R 1 9 5 1
The measurement of the refractite index of elastomers is discussed, with particular reference to GR-S rubber and the use of an Abbe-type refractometer. Detailed descriptions of the apparatus used and the procedure for GR-S rubber are given. Common sources of error are discussed and methods of minimizing these errors are gi\en. A detailed study has been made of the precision obtainable with GK-S polymers using the Abbe-tj pe refractometer with an incandescent light source. It was found that the standard deviation corresponding to variabilitj of a single random determination w-as 0.00007. The method has been used for seieral years as the basis for the determination of bound styrene in copolymers of butadiene and styrene and is currently employed for control purposes in the governmentowned synthetic rubber plants. It has also been used for determining the per cent of rubber hydrocarbon in natural rubber and for locating second-order transition temperatures.
for checking the scale setting are frequently marked inaccurately, sometimes til- as much as 0.0003. This is of no consequence if the instrument has been calibrated with the particular Glass test piece by a standardizing laboratory arid the corrections furnished by that laboratory me applied t o each reading. I n the absence of such a calibration a standard glass test piece with a n accurately known value not too far ii,om that of the rubber may be used. The procedure, as given beloiv, provides for a check of the scale of the instrument with the standard glass test piece and a readjustment if necessary. Xormally this adjustment should be required only a t intervals of weeks or months. If i t is necessary to makr the adjustment frequently, either the instrument is being subjrcted t o severe jars or vibrations or there is a n imperfect connection permitting relative motion of the prism and index nrn1. E'oI the preparation of the specimens it is necessary t o have available a mill of some sort for sheeting out the rubber, conventional apparatus for estraction of nonrubber constituents, and a hydraulic press. The extractant is the ethyl alcoholtoluene azeotrope-70% ethyl alcohol and 30% toluene by voluIne-commonly used with GR-S and designated as E-T-A. The press should be capable of being heated t o 100" C. and should furnish a force of 50 to 300 pounds for each specimen-for example, if I O speciniens of 1 square inch each are pressed a t the same time, a force of 500 t o 3000 pounds is used. The resulting specimen thickness is usually between 0.010 and 0.030 inch (0.025 and 0.075 cm.) but is not at all critical for the measurement,. I t is often difficult t o obtain good optical contact with the prism when the specimens are appreciably thicker than the upper value given. PROCEDURE FOR G R - S
The procedure used for GR-8 synthetic rubber is described in considerable detail. It can readily be modified for measurementh on other types of polymers.
A rubber sample of about 2 grams is first blended by passing i t through a mill several times, sheeted as thin as possible, so t h a t it is not over 0.020 inch (0.05 cm.) a t any point, and cut into strips 1 cm. wide and 2.4 cm. long. Each specimen is threaded on a separate Kchrome wire fastened t o each corner of I-cm. square of aluminum sheet. The specimens are then placed in a 400-ml. extraction flask containing 60 ml. of E-T-A and extracted under gentle reHux for 2 hours with one change of solvent a t the end of the first hour. I n the case of alum-coagulated rubbers (4)E-T-A containing 10 ml. of concentrated hydrochloric acid per liter of solution is used. When acidified E-T-A4 is used, each specimen should be fastened on noncorroding wire attached t o squares of noncorroding materid such as tantalum or plati-
1657 num. After extraction the specimens are removed from the flask and dried in a vacuum oven a t 100" C. for 1 hour. The dried strips are removed from the wire and each specimen is placed between two 1-inch squares of clean aluminum foil. The specimens are pressed with a force of 50 t o 300 pounds per specimen and a t a temperature of 100" C. for 3 to 10 minutes, depending on the amount of gel present in the specimen. After hot-pressing, the specimens are cooled under pressure either by circulating cold water through the press platens, or by using a press equipped with both hot and cold platens. For measurements of index with the Abbe refractometer, the instrument is placed with the telescope a t a 45" angle with the vertical, on the opposite side of the vertical from the position employed during the measurement of the index of liquids. The hinged illuminating prism is not used and consequently is rotated away from the measuring prism. The refractometer is checked a t least once a day against the standard glass test piece. The test piece as well as the prism is cleaned Ivith alcohol applied with lens paper both before and after the adjustment of the instrument is checked. Acetone should not be used, because it attacks the cement holding the prism in place. The instrument is illuminated with a flashlight bulb or a n automobile lamp not exceeding 3 candlepower, and a broad diffuse light source is obtained by merely covering the bulb with crumpled tissue paper. The position of the light source is adjusted t o give the most distinct dividing line. A small drop of 1-bromonaphthalene is placed on the polished flat surface of the test piece and the latter is pressed firmly against the measuring prism of the refractometer with the polished end to\vard the light. A minimum amount of 1-bromonaphthalene is used, so t h a t when the test piece is pressed against the prism surface the liquid film does not flow beyond t,he polished end of the test piece and does not form a wedge of liquid between the face of the test piece and the surface of the refractometer priani. The surface of the prism must, be completely free of contact liquid between the test piece and the light source. If these precautions are not taken, a line may be observed a t a position different from that corresponding to the indes of the glass test piece and false settings made. The boundary between light and dark portions of the field of the telescope is observed by setting the compensator drum, which rotates the Amici prisms used for achromatization, a t one of the two positions that give a boundary line most nearly free from color. The contact between the surface of the test piece and of the prism is tested by firmly pressing the two surfaces together with the finger. If there is no displacement of the boundary line during this operetion, the position of the slab may be considered sat,isfactory. Three readings are made without moving the test piece. Then the test piece is removed, and replaced on the prism, and three additional readings are made. If the mean of these readings on the test piece is different from the marked value., the scale of the instrument is set a t the marked value ant1 thc dividing line is brought into coincidence with the cross hairs by turning the small screw that is recessed in the outer barrel of the telescope. .liter the adjustment the operat,ion of checking the instrument is repeated. Before the glass test piece is removed or the scale setting is disturbed, the Compensator drum is turned to the second position which gives a boundary line frce from color. If the cross hairs are found t o be displaced from the boundary line by more than 0.0002, the compensator asymmetry is too large to be neglected, and the refractometer should be returned t o the maker for adjustment; however, such errors may be minimized by using corresponding achromat,izing positions for both the test piece and specimen. The tissue paper is removed from the light bulb, and a tube about 2 inches long is made by rolling up a piece of aluminum. The tube is slipped over the bulb to provide a beam of light at, grazing incidence t o the prism. Refractive index measurements are then made on the pressed samples of rubber. The "sandwich" of aluminum foil and rubber is first cut in half. Then the narrow end of one of the pieces of aluminum foil is peeled off with a quick continuous pull, leaving the rubber adhering to the other piece of foil. Wit,h a razor blade a strip is cut a t least 0.3 cm. wide and 1 cm. long where the GR-S is smoothest and completely free from air bubbles. I t is necessary to obtain a sharp clean edge perpendicular t o the plane of the foil. The specimen is placed on the prism of therefractometer with the cut edge away from the observer, perpendicular t o the length of the prism face, and in about the same position a: that previously occupied by the polished edge of the glass test piece. I t is necessary t o determine by experiment for each refractometer the proper location of the cut edge along the prism face. This location is t h a t which will give the best field in the telescope; it has been found t o be about 0.125 inch below the center of the prism face in several instruments and slightly above
1658
ANALYTICAL CHEMISTRY
the center in others. T h e specimen is pressed firmly with a finger against the foil until it wets the glass, leaving no air entrapped between the prism and the specimen near the cut edge. No use is made of the refractometer's illuminating prism. Water is circulated a t room temperature through the housing of the measuring prism of the refractometer and through the cored brass block which is now placed over the specimen. S t least 1 minute is allowed to elapse for the specimen to reach temperature equilibrium after the block is placed over the specimen and before a n index reading is made. The compensator is adjusted to give a boundary as free from color as possible, and the index is read by setting the boundary between the light and dark portions of the field on the intersection of t,he cross hairs. For all normal GR-S polymers the boundary between light and dark portions of the field should be as sharp as the boundary obtained with liquids. I n reading the line the compensator drum is adjusted so that of the two possible popitions of compensation the one used for the rubber specimen is nearer t o that used for the glass test piece. The setting is approached from a position in which the intersection is in the light portion of the field. Bt least three settings are made and if there is a difference of more than 0.0001 between the readings a new strip is mounted on the prism and the readings are repeated. If average values obtained on different specimens from the same sandwich differ by more than 0.0002, a new portion of the original sample is extracted and the observations are repeated. .4t each reading the temperature of the circulating water is recorded to the nearest 0.1" C. T h e value of 1 ~ 2 5 , the index a t 25" C., is computed from each observation by use of the equation 7126 = nr 3 . i X lo-' ( t - 25) where I t l is the index observed a t temperature t. I n other wordfi, the observed value is increased by 0.00037 for each degree by which the temperature exceeds 25" C., or conversely ( 1 ).
+
SOURCES OF ERROR
To obtain a sharp boundary line in the field of the refractometer it is necessary to follow the foregoing procedure fairly closely. If the sample is not homogeneous, it should be thoroughly blended by milling or other suitable means. No change in index has been observed on prolonged milling. If the surface of the specimen is rough or if it contains air bubbles, it will not adhere smoothly t o the measuring prism. Sormal GR-S, essentially free from gel, requires about 3 minutes of pressing a t 100" C. under a pressure of 300 pounds per square inch. If the sample contains a large amount of gel or has a very high styrene content, longer times may be required. Finally, the specimen must have a sharp clean-cut edge perpendicular t o the face of the prism.
I
2
3
4
5
6
7
8 9 1011 121314 15
DAY Figure 1.
Effect of Day-to-Day Variability on Reproducibility of Test
Even where a sharp line is readily obtained, the observed value may not represent the desired index. For example, the observed value will not represent the index of the polymer itself unless all interfering impurities are removed. Extraction, under the conditions already described, is adequate t o accomplish this under conditions normally encountered. I n the case of rubbers coagulated with alum, the aluminum soaps formed are only
sparingly soluble in alcohol. Dryden ( 4 )showed, however, that if water is present in the E-T-A these soaps a r e hydrolyzed to a large extent to aluminum hydroxide and fatty acid. The latter is extracted and the former, being insoluble in GR-S, does not affect its index. If hydrolysis is performed in the presence of a small amount of mineral acid as recommended in the present procedure, the reaction goes t o completion.
Table I. Day 1
2 3 4
5 6
7
Individual Values of Refractive Index, n y
Flask 1 1.5350 1.5349 1,5350 1,5351 1 5349 1.5351 1.5351 1.5350 1 5351 1.5381 1.5351 1 5351 1.5351 1.5351 1.5350 1.5319 1.5350 1 5351 1.5351 1,5351 1,5360
Flask 2 1.5319 1.3349 1.5349 1 5349 1.5352 1.5352 1.5351 1,5351 1.5351 1.3351 1,5351 1.5351 1.5361 1.5351 1.5350 1,5350 1,5351 1.6350 1.5351 1,5350 1,5351
Day 8 9 10 11 12
13 14 15
Mean. High. Low. si. s/. Sd. SI.
Flask 1 1 5351 1.5351 1.5351 1,5352 1,5352 1,5352 1.5351 1.5361 1,5350 1.5351 1.5351 1,5352 1.5351 1.5351 1,5382 1.5351 1,5351 1.5351 1.5351 1.5351 1.5351 1.5351 1,5350 1.5351
Flask 2 1 5351 1 5351 1 3350 1 ,6361 I ,6351 1.3331 133.51 1.5331 1,5331 1 . ,5330 1.R35l 1,3351 1 . 3350 1,3351 1.5351 1.5351 1.5351 1 ,6351 1.2351 1.6361 1 . ,5351 1 3351 1 ,5351 1.3330
1.53507 1.5352 1.5349 0.00006 0,00000 0,00004
0.00007
0.000012 Standard deviation correipunding to intrinsic variability. 8,. s/. Standard deviation corresponding t o flask-to-flask variability. sd. Standard deviation corresponding to day-to-day variability. sr. Standard deviation of a single random determination. em. Standard deviation of the mean of 90 determinations. s,n.
It has been found that residual impurities amounting to 0.14% sodium stearate, 0.14% stearic acid, or 0.07% ethyl alcoholtoluene azeotrope will decrease, or 0.07% phenyl-2-naphthylamine will increase the index by 0.0001 (11). It is therefore necessary t o extract completely all extraneous material. The work o f Custer ( 3 ) showed that several standard lots of normal GR-S changed in index by not more than 0.0001 in 2 years of storage a t room temperature in the dark, Ahereas samples held for 4 weeks a t 70" C. in air increased in index by about 0.0005. T h e change is probably associated with incomplete extraction of phenyl-2naphthylamine or ite oxidation products from the rubber after the accelerated aging. After extraction, the specimen should be exposed to osygen as little as possible, because the extraction removes all antioxidants. Besides the increase in index caused by the actual addition (,f the oxygen t o the polymer, oxidation also makes it more difficult t o obtain a good line of demarcation in the refractometer. T o avoid oxidation as much as possible during drying, it is suggested that all the residual solvent be removed from the specimen under vacuum. Finally, there is the important matter of accuracy in the measurement of temperature of the specimen. The rate of change of' index of normal GR-S with temperature is about -3.7 X lo-' per degree centigrade (1)and is essentially the same as for natural rubber. This is so large that one must be sure of temperature equilibrium, equality of temperatures of water and specimen, and accuracy of the thermometer. PRECISlON
A statistical analysis was made on refractive index data obtained in.order t o determine the precision of the measurements.
1659
V O L U M E 23, NO. 11, N O V E M B E R 1 9 5 1 These data were obtained by extracting three specimens of X-452 GR-S in each of two flasks on 15 different days. T h e refractive index of each specimen was determined and corrected to 25” C. by the temperature coefficient, given above. The data are presenkd in Table I. I n Figure 1 the ordinate is the average of the six measurements made on each day. T h e solid line represents the mean of the ninety determinations made during the 15 days, and the dotted lines represent the maximum range t o be expected if there is no day-to-day variation. All but two of the measurements fall well within these lines;.
determination (95 out of 100 random observations may be espected to fall within a range of 2sl around the mean value); and ( 5 ) s,,,, the etandard deviation of the mean of all 90 determinations appearing in Table I. T h e latter standard deviation was obtained by the following formula: sm = (st2/90
+ si2/2 + ~ 2 / 1 5 ) ’ / ~
where the divisors 90, 2, and 15 refer, respectively, to the 90 individulrl determinationq, the 2 flasks, and the 15 days involved in the entire experiment. LITERATURE CITED
(1j .Irnold, Aurelia, and Wood, L. A , ”Rate of Change of Refractive
Y)
0 X
$ 0
c
a
I
2
3 4 5 6 7 8 9 IO II 12 13 1415 DAY
Figure 2.
Effect of Flask-to-Flask Variability on Reproducibility of Test
Figure 2 shows the differences: between the two flask averages T h e solid line in the center represents their mean. It’can readily be seen that there are no appreciable differences between estractions. An analysis of variance was made on these data (9)and from it five standard deviations w x e calculated: (1) si, the standard deviationcorresponding t o intrinsic variability (the error reflected by the variability of replicate determinations made in the same flask by the same operator a t approsimately the same time): ( 2 ) SJ, t,he standard deviation corresponding t o flask-to-flask variability (the additional variability introduced when replicate deterniinations are made in different flasks); (3) a d , the additional standard deviation corresponding t o day-t,o-day variability: ( 4 ) sr, the standard deviation which might he expected for R single random
Index of GR-S with Temperature,” unpublished report to Office of Rubber Reserve, Feb. 28, 1949. (2) .irnoid, Aurelia, and Wood, L. A , “Simplified Procedure for the Determination of Bound Styrene in GR-S by Measurement of Refractive Index,” unpublished report to Office of Rubber Reserve, March 4, 1949. (3) Custer, Patricia, “Effect of Aging on the Refractive Index of GR-9,” unpublished report to Office of Rubber Reserve, Dec. 10, 1946. (4) Dryden, H. L., Jr.. “Extraction of Alum-Coagulated GR-S for Refractive Index Determinations,” unpublished report to Office of Rubber Reserve, ,July 9, 1945. (5) Fanning, R. J.. and Bekkedahl, Korman, .%NAI.. CHEM.,23, 1653 (1951). (6) McPherson, h. T., and Cumniings, A. D., .I. Research .VatE. R ~ L TStandards, . 14, 553 (1935). RP786; Ruhbar Chem. and Technol., 8 , 421 (1935). ( i )Madorsky, Irving, and Wood, L. “Measurement of Refractive Index and Determination of the Styrene Content of GR-S Copolymers,” unpublished report to Office of Rubber Reserve, Sept. 13, 1944. (8) Madorsky, Irving, and Wood, L. A , , “Procedure for the Measurement of the Refractive Index of Specification GR-S,” unpublished report to Office of Rubber Reserve, N o r . 30, 1944. (9) Jnedecor, G. W., “Statistical Methods,” 4th ed., p. 238, Tables 10-16, ..imes, Iowa, Iowa State College Press, 1946. (10) IViley, R. H., Brauer, G. XI., and Bennett, .i. R., .I. Polymer Sci.. 5 , 609 (1950). (11) 11-ood, L. .I.,unpublished work a t Sational Bureau of Htandards. Aiiril 5 , 1931. I’rewnted before t h e 58th meeting of t h e Division of Rubber Chemi$try. . ~ \ I E R I C A S C H E V I C A SOCIETY, L Washington, D. C., M a r c h 2 . 19S1. RECEIVED
Estimation of Sulfur in Petroleum Products Electrical Device f o r Use in Determining Sulfur b y the Lamp Method C . W. BROWN Research und Development D e p a r t m e n t , Socony- V a c u u m Laboratories, Paulsboro, N. J .
S
L-LFUH. in gasoline and related light products is usually dr-
termined by burning a sample in a wick-fed lamp, absorbing the combustion gases in a peroxide solution and measuring the resultant sulfate ion in the absorbing solution either volumetrically ( 1 , 3 ) or gravimetrically ( 2 ) . The volumetric procedure has heen more generally used because it is simpler and Icss timc-milPuming although it may be slightly less accuratr. SIZE O F SAMPLE
A major problem that has always confronted operators of the lamp test for sulfur has been t h e ever-present uncertainty of knotring how much sample to burn. It has been customary, berause of this uncertainty, to burn :i relatively large sample in order to produce sufficient acid for :t suitable titration with the conventional 0.0624 .J7 sodium hydroxide solution. This procedure is fairly satisfactory when a fuel of low sulfur content is being
test,ed, but when a sample contains relatively high sulfur, the sulfuric acid formed in the absorbing solution is high and this necessitates a long painstaking titration, Because much time could be saved if the “end point” of thc titration c,ould be estimatcti closely, it seemed highly de~ir:ihle to find a w i y to pre:lict Lvithin rcasonable tolerance the amouiit of sulfuric acid formed in the ahsorbing solution ( t hi$ presupposes fuels containing only carbon, hydrogen, and sulfurj. SOattempt was made a t this time t o provide a substitute for the conventional volumetric titration procedure, but merely to institute a means for checking the acid content of the absorbing solution a t any time during the sample-burning period. Of three possibilities for accomplishing this end result, the third was selected as offering the most promise and was developed for incorporating into the routine procedure.
